The proposed research is an attempt to characterize the transport pathways for water and solutes across the epithelium of renal tubules. Electrophysiological techniques will be used in order to assess the permeability properties of the intercellular and the transcellular pathway crossing the tubular wall. Furthermore experiments will be performed in order to explore the intraepithelial route followed by water and how it is coupled to primary solute flow. The characteristics of active transport mechanisms at the single membrane level will be studied by means of microelectrophoretic injections of ions within the renal cell. Finally, the possibility of some interaction between different solute flows across the epithelium will be tested.